1. Field of the Invention
The present invention relates to a Fresnel lens having excellent optical properties and impact resistance.
2. Description of the Related Art
Rear projection screens are typically used for projecting expanded pictorial images from televisions or films in rooms having a significant amount of ambient light. The rear projection screens condense and diffuse incident light beams from a CRT (cathode ray tube) to regulate the optical path of the light beams so as to allow pictorial images to be sufficiently visible. As the rear projection screen, a screen comprising a Fresnel lens and a lenticular lens is generally used.
As the Fresnel lens has a number of grooves on the surface thereof, which are shaped during manufacturing, it is accordingly not made of glass but is conventionally made of a transparent resin, such as polystyrene or polymethyl methacrylate. Though polystyrene is a preferable material, having a large refractive index, the insufficient strength of polystyrene makes difficult the manufacture of large sized lenses from this material. Moreover, Fresnel lenses may be chipped or cracked by impact during manufacture or transportation. Such problems tend to arise especially in Fresnel lenses made of a styrene resin having a high refractive index.
Fresnel lenses made of cross-linked polystyrene have been proposed as a way of improving these disadvantages; for example, UNEXAMINED JAPANESE PATENT PUBLICATION No. 61-223701 discloses a synthetic resin lens prepared by polymerizing a styrenic monomer of not less than 0.4 weight fraction and a cross-linking agent of from 0.001 to 0.3 weight fraction.
While the disclosure in UNEXAMINED JAPANESE PATENT PUBLICATION No. 61-223701 was intended to improve the impact resistance of the lens by cross-linking the resin, it does not succeed in obtaining practically sufficient impact resistance.